When replacing the oil pan gasket the side gaskets must be stuck up onto the block first and then the end corks inserted into the main bearing caps. Pre-curl the end corks before installing. Saves a lot of swearing. I uses 4 little studs , insert them into the block, to guide the pan straight up into place. This helps to prevent the gaskets from slipping.
I just finished installing the oil pan gaskets with the engine out of the car and turned over (oil pan up). So I would like to add to Gene's above comments. I was surprised that the cork was about an 1/8th to 1/4 inch too short. I had purchased the gaskets from Chevy's of the 40s so knew I had a good product. This should have been an easy install with everything so easy to get to and put into place. I started by putting one end of the gasket on top of the side gasket then trying to get it to set into its groove. But this even seemed to be a problem, the gasket seemed to be to wide. I changed techniques and lined the gasket up with the bearing cap inner edge in the corner then used a large round edge on a punch (1/4 inch in diameter)to force it flat into its groove. I worked slowly from one corner to the other. Things were now going well but the gasket was still too short. I started over several times and began to notice that the gasket was stretching so that on my fourth or fifth attempt it finally aligned tightly on the side gaskets. I did things a little different on my second end gasket, I played with the gasket trying to stretch it a little before installing it ("precurling it," as Gene said). This sped the process up and on my third attempt I had the gasket tight against the side ones. The side gaskets were now so tight that they needed a slight pull to release them. I put a small amount of gasket sealer on both ends of each side gasket and carefully forced them back under the end gaskets and then began the process of gradually seating the oil pan onto the block. Lots of small tightening of bolts and screws to gradually draw everything into place.
At this point I am in even greater admiration of the guys that do this with the engine in their vehicles, and even of the fact I had done it that way the last time I had put a new gasket on my oil pan 5 years ago (
that only leaked a little in the corners). Probably caused by not stretching the gasket enough to tightly seal each corner, even with using sealant in the corners. In the past I had also used a dab of superglue, every so many inches, to keep the gasket suspended while working on the end gaskets and placing the oil pan in place. Good luck, Mike